The present invention relates to a motor control method, and more particularly to a method of controlling the rotation of a motor with a deviation signal produced by comparing a reference period signal and a signal generated by a rotational speed detector coupled to the motor as it rotates at an ultra low speed, the generated signal representing the period between adjacent positive-going edges and the period between adjacent negative-going edges of a pulse train in each of a plurality of motor phases.
For performing highly accurate control in servo systems, it has widely been practiced to attach a tachometer generator to the rotating shaft of a servomotor to provide a feedback loop, feed back an output signal indicative of the voltage from the tachometer generator through the feedback loop, and compare the signal with a control reference signal for controlling the rotation of the servomotor.
The servo system for controlling the rotation of a servomotor at an ultra low speed would considerably be expensive if it incorporated a tachometer generator for producing a feedback signal. To avoid this drawback, there have been developed and used various devices in which the servomotor is associated with a relatively inexpensive rotational speed detector such as a rotary encoder, rather than the tachometer generator, to produce pulses which are counted for digital feedback control of the servomotor.
In the devices in which the servomotor is required to be driven at an ultra low speed such as 0.3 through 0.5 revolution per minute, however, the pulses produced by the rotary encoder are quite few, and it would be difficult from the standpoint of accuracy to effect feedback control with the few pulses from the rotary encoder. One solution would be to use a high-frequency rotary encoder capable of producing many pulses when the motor rotates at an ultra low speed, but such a highfrequency rotary encoder would be very expensive to manufacture.